The formation of a formalin-fixed paraffin-embedded (FFPE) tissue block serves to preserve the morphology and cellular content of a tissue sample. Tissue processing generally involves placing an isolated tissue in formalin for a time period such as a few days, and then embedding the tissue in a paraffin wax. FFPE samples can be conveniently stored at room temperature for extended periods of time, and are especially useful for immunohistochemical staining and morphology analyses. FFPE samples may also be used for profiling gene expression and studying diseases.
At the time of biological testing, the FFPE tissue block is generally trimmed by cutting the tissue block on a microtome. The tissue block may be analyzed to determine the boundaries of the tissue in the FFPE by a technician or using an automated method. In the former case, a technician generally examines the FFPE block to observe the diffuse image of the tissue embedded in the paraffin. The technician may ascertain what the cross-sectional area of a section comprising the tissue should look like and compare that to the tissue sections as they emerge from the microtome blade. Preferably, the tissue block is trimmed to expose a representative amount of tissue to the surface of the block and to ensure that the block face is in line with the knife's edge.
During automated analysis, a camera is commonly utilized to image the tissue. A light source illuminates the surface of the tissue block at an angle to distinguish the difference between the paraffin and tissue surfaces. Since paraffin is comparably smoother than tissue, automated analysis utilizes the different natural textures of paraffin and tissue to differentiate between the two materials.
US 2010/0118133 A1 discloses an automated method and device for producing thin sections of tissue and obtaining an image of a surface generated by sectioning the sample using a camera. A device is used to evaluate the image to determine whether the section of the sample is acceptable for further use.
Many existing methods provide inaccurate and inconsistent data when used to analyze different tissue and paraffin types, since many methods are sensitive to variability of optical and surface characteristics of tissue and paraffin. In some cases, it is quite difficult to distinguish tissue from paraffin in an FFPE sample using existing methods.
Accordingly, there is a need for an additional method and apparatus for determining the location of a tissue sample in an embedding medium such as paraffin. The present methods and apparatus provide an accurate and consistent method for differentiating tissue from paraffin in a tissue block.